The present invention generally relates to light sources, and more particularly, relates to photon sources used for cryptography and other applications. This invention was made with Government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.
Entangled states of multiparticle systems are arguably the quintessential feature of quantum mechanics. These systems form the basis of quantum information, and enable such phenomena as quantum cryptography, dense coding, teleportation, and quantum computation. At present, the most accessible and controllable source of entanglement arises from the process of spontaneous parametric down-conversion in a nonlinear optical crystal. Herein is described a realization of an ultrabright source of polarization-entangled photon pairs, using two such nonlinear crystals. This was first described in the publication xe2x80x9cUltrabright Source of Polarization-Entangled Photons,xe2x80x9d P. G. Kwiat, E. Waks, A. G. White, I. Applebaum, and P. H. Eberhard, Phys. Rev. A, 60, R773 (1999). Because, in the present invention, nearly every pair of photons produced is polarization-entangled, the total flux of emitted polarization-entangled pairs should be hundreds of times greater than is achievable with the best previous source, for comparable pump powers. The new technique has the added advantage that the degree of entanglement and the purity of the state are readily tunable, a heretofore impossible feature.
It is now well known that the photons produced by way of the down-conversion process share nonclassical correlations. In particular, when a pump photon splits into two daughter photons, conservation of energy and momentum lead to entanglements in these two continuous degrees of freedom, as has been demonstrated in various tests of Bell""s inequalities, showing that nature is nonlocal. Yet, conceptually, the simplest examples of entangled states of two photons are the polarization-entangled xe2x80x9cBell states:xe2x80x9d
|H1,V2xc2x1|V1,H2 greater than ; |H1, H2xc2x1|V1,V2 greater than ;
where H and V denote horizontal and vertical polarization, respectively. For convenience, the normalization factor (1/2) is omitted. To date there have been only two methods for producing such polarization-entangled photon pairs, and each has fairly substantial limitations.
The first method was an atomic cascade: a two-photon decay process from one state of zero angular momentum to another. The resulting photons do display nonclassical correlations, and were the photons used in the first tests of Bell""s inequalities. However, these correlations decrease if the photons are not emitted back-to-back, as is allowed by recoil of the parent atom. This correlation problem was circumvented with parametric down-conversion, since the emission directions of the photons are well correlated.
The second method provides a source of truly polarization-entangled photons, and was realized using down-conversion with type-II phase matching, in which the photons are produced with definite orthogonal polarizations. For two particular emission directions, however, the correlated photons are produced in the state, HV+VH, and additional birefringent elements in one or both emission directions allow the formation of all four Bell states. This source has been employed to demonstrate quantum dense coding, teleportation, a post-selection-free test of Bell""s inequality for energy and time variables, a test of Bell""s inequality (for polarization variables) free of the usual rapid-switching loophole, and most recently, the generation of GHZ states of three photons. Coincidence count rates of up to xcx9c2000 sxe2x88x921 (for a 3-mm thick BBO crystal and a 150 mW pump) have been observed with this source, while maintaining an acceptable level of entanglement.
Nevertheless, the brightness of this prior art source is still very limited because the photons are polarization-entangled only along two special directions. The present invention is capable of providing a bright source in which polarization-entangled photons are produced along all or nearly all emission directions.
It is therefore an object of the present invention to provide a bright source of polarization-entangled photons.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus of this invention comprises a laser for emitting polarized laser light, with at least two nonlinear crystals having optical axes positioned adjacent to any neighboring of the at least two nonlinear crystals, each of the optical axes forming a predetermined angle with adjacent optical axes of the at least two nonlinear crystals. The at least two nonlinear crystals receive the polarized laser light, and output light containing polarization entangled photon pairs.
In a further aspect of the present invention, and in accordance with its objects and principles, apparatus for producing a bright source of polarization entangled photons comprises a laser emitting laser light, with a polarizer in optical communication with the laser for polarizing the laser light. At least two nonlinear crystals having optical axes are positioned adjacent to any neighboring of the at least two nonlinear crystals, each of the optical axes forming a predetermined angle with adjacent optical axes of the at least two nonlinear crystals. The at least two nonlinear crystals receive the polarized laser light, and output light containing polarization entangled photon pairs.
In a still further aspect of the present invention, and in accordance with its objects and principles, a method for producing a bright source of polarization entangled photons comprises the steps of directing polarized laser light to at least two adjacent nonlinear crystals having optical axes, wherein each of the optical axes forms a predetermined angle with adjacent optical axes of the at least two adjacent nonlinear crystals; and outputting light from the at least two adjacent nonlinear crystals, the light containing polarization entangled photon pairs.
In a yet further aspect of the present invention, and in accordance with its objects and principles, a method for producing a bright source of polarization entangled photons comprises the steps of generating laser light; polarizing the laser light; transmitting the polarized laser light to at least two adjacent nonlinear crystals having optical axes, wherein each of the optical axes forms a predetermined angle with adjacent optical axes of the at least two nonlinear crystals; and outputting light from the at least two adjacent nonlinear crystals, the light containing polarization entangled photon pairs.
In still another aspect of the present invention and in accordance with its objects and principles, apparatus for providing a bright source of polarization entangled photons comprises a laser emitting laser light and means for directing the laser light to at least two nonlinear crystals, each of the at least two nonlinear crystals having optical axes forming a predetermined angle with the optical axes of other of the at least two nonlinear crystals, with each of the at least two nonlinear crystals outputting cones of polarized pairs of photons. Superposing means receive the cones of polarized pairs of photons for superposing the cones of polarized pairs of photons, so that light output from the superposing means contains entangled photon pairs.